Abstract

The niches of herbivorous insects are best understood in a tri-trophic perspective, as top-down effects of predators and parasitoids can determine the host plant range of herbivores. The recent introduction in a tropical agricultural environment of a weedy open-habitat plant (Solanum myriacanthum) and subsequent host range expansion of a common forest-edge butterfly (Mechanitis menapis) onto that plant provides an opportunity to test hypotheses surrounding reconfiguration of tritrophic networks in anthropized environments: what is the role of bottom-up and top-down forces in this host range expansion? and does habitat breadth limit this novel trophic relationship? Field and laboratory monitoring of larval survival and performance on a native (Solanum acerifolium) and exotic host in the Mindo valley (Ecuador), combined with measurement of plant physical defenses, shows that larval mortality was mostly top-down on S. acerifolium, linked to parasitism, but mostly bottom-up on S. myriacanthum, possibly linked to observed increased plant defenses. These findings support the theory that herbivores experience little top-down regulation on exotic hosts, and suggest that, in this case, bottom-up pressure from plant defenses is stronger in the absence of a co-evolved relationship. In this system, top-down forces tend to expand rather than restrict host plant range, contrary to many previous studies. S myriacanthum was less colonized in open pastures than in semi-shaded habitats (forest edges, thickets): fewer eggs were found, suggesting limited dispersal of adult butterflies into the harsh open environments, and the survival rate of first instar larvae was lower than on ecotone plants, likely linked to the stronger defenses of sun-grown leaves. Environmental conditions thus modulate the rewiring of novel trophic networks in heavily impacted landscapes, and limit a native herbivore`s control on an invasive plant.